Photoelectric sensors, including modulated photoelectric sensors, are being used more frequently in safety related applications. The primary example is in safety light curtains, sometimes referred to as machine guards. These safety light curtains comprise multiple beams of electromagnetic energy, typically infrared modulated light, which cross an access area to a dangerous machine such as a punch press. If an operator's hand or arm, for example, penetrates the curtain, the machine is instantly shut down to prevent injury.
These types of devices are known in the art. Regulatory agency standards exist for these devices, and the standards are continually being refined. One common theme in all safety standards is a determination of the effect (if any) of component failure. The requirement is that the circuit fail to a safe condition, regardless of which component or plurality of components fail, or in the mode in which they fail. This requirement becomes particularly constraining because if a component failure goes "unannounced," the failed component must be left in the faulty state, and the failure analysis proceeds with other components. The designer is thus saddled with a "who watches the watchman" requirement.
The term that is often used to describe this type of analysis is "FMEA" (Failure Mode Effects Analysis). In this type of testing, each component is alternately opened and shorted. The effect on the circuit output of opening and shorting circuit components is analyzed to ensure that in no case does the safety circuit fail to a dangerous condition.
The photodetector itself and its associated amplifier circuitry is a particularly vulnerable portion of a photoelectric circuit. Designing a safe circuit is made especially difficult because the signals received by the photodetector are very small, often resulting in a generated signal of less than one millivolt, and the modulation or "pulsing" frequency of the received signal is high, often many KHZ. Compounding the problem is the system's pulsing of a group of light emitting diodes (LEDs) in order to transmit the high frequency modulated "light" signals for the beams of the light curtain. The receivers are designed to be most sensitive to, and have the highest gain at, the pulsing frequency of the LEDs. Therefore, if the emitting circuit induces any noise on the power supply, the noise may be coupled to the receiver circuit amplifier, potentially causing the receiver circuit to enter a failure mode in which the noise falsely appears at the receiver amplifier output as a valid "light" signal. This failure mode is especially likely when the amplifier circuitry gain is very high.
Another problem occurs when component failure creates positive feedback, resulting in oscillations within the amplifier circuit. High gain amplifier circuits are prone to go into oscillation any time positive feedback occurs. The frequency of the faulty oscillation is generally high compared to the modulation frequency of the received light. A component or harmonic of the faulty oscillation is frequently at just the right frequency to "fool" the receiver into responding as if it has received a valid "light" signal from the emitter. Accordingly, when component failure results in high frequency noise in the amplifier, the circuitry coupled to the amplifier output may be "fooled" into operating as if the amplifier has received "light" pulses, when in fact is has not.
This is the most dangerous potential condition. In this condition, a person's hand or arm could be blocking a "light" beam, but the receiver operates as if it is receiving an unblocked "light" signal, which creates the very dangerous situation of allowing the machine to operate even though the person's arm or hand has penetrated the light curtain.
A need, therefore, exists in the art for a photoelectric receiving and amplifier circuit that will not oscillate nor create false pulsing outputs no matter what failure mode exists in any of its components.